Motor system with voltage limiting feedback



Aug. 10, 1948. A. P. UPTON MOTOR SYSTEM WITH VOLTAGE LIMITING FEEDBACK Filed Oct. 14, 1942 Gttomeg Patented Aug. 10, 1948 MOTOR SYSTEM WITH VOLTAGE LIMITING FEEDBACK Albert P. Upton, Minneapolis, Minn assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation oi. Delaware Application October 14, 1942, Serial No. 461,955

10 Claims.

The present invention relates to electronic amplifiers, and especially to amplifiers adapted for use in control apparatus.

An object of the present invention is to provide an improved control system of the type wherein an alternating potential is varied in accordance with a controlling condition, amplified, and the amplified potential is used to control recording, indicating, or controlling apparatus.

Another object of the present invention is to provide an improved amplifier circiut for such a control system.

A further object is to provide, in such an amplifier, means for substantially preventing a shift in phase between the signal applied to the amplifier input terminals and the alternating electrical energy available at the amplifier output terminals. i

A further obiectof the present invention is to provide an improved control system of the balanced type, wherein a given change in a controlling condition produces a corresponding change in the condition of a controlled device, and wherein means are provided to balance an effect corresponding to the controlling condition against an effect corresponding to the condition of the controlled device.

A further object of the invention is to provide, in such a balanced control system, improved means for preventing hunting of the control system. I

A further object of the present invention is to provide an electronic amplifier circuit including improved current limiting means of the automatic volume control type.

Other objects and advantages of my invention will become apparent from a consideration of the appended specification, claims and drawing, in which:

Figure 1 is an electrical wiring diagram of an amplifier circuit embodying the principles of my invention, and

Figure 2 is an electrical wiring diagram of a temperature control system adapted to utilize the amplifier circuit shown in Figure 1.

Referring now to Figure 1, there is shown an electronic amplifier, generally indicated at I6, having input terminals H and I2, output terminals i 3 and I4, and power supply terminals l5 and I6. The amplifier HI includes a first voltage amplification stage generally indicated at 26, a second voltage amplification stage 2|, and a final power amplification stage 22. The amplifier III also includes a power supply circuit 23 o! the conventional full wave rectifier type.

The power supply circuit 23 includes an electric discharge device 24 of the twin diode type, having anodes 25 and 26 cooperating with a common cathode 21. The anodes 25 and 26 are connected by conductors 28 and 29, respectively, to the opposite terminals of a secondary winding 30 on a transformer 3| having a primary winding 32. The primary winding 32 is connected directly to the power supply terminals l5 and 16 of the amplifier ID. The cathode 21 is indicated as being of the directly heated filament type, and is supplied with electrical energy for heating purposes frorn another secondary winding 33 on the transformer 3|. The center tap 34 of secondary winding 30 is connected through a conductor 35 to a conductor 36, which is grounded at 31.

Cathode 21 is connected to a conductor 40.

The circuits to which power is supplied by the rectifier circuit 23 are connected between the conductors 36 and 40. It will be seen that the secondary winding 30 applies a positive potential alternately to the anodes 25 and 26, and that the current fiow thereby resulting in the load supplied by the power supply circuit 23 is in a direction such that conductor 40 is positive with respect to conductor 36, as indicated by the legend in the drawing. A filter condenser 4| is connected between the conductors 36 and 40. A filter network comprising resistors 42 and 43, and condensers 44 and 45, also interconnects the conductors 36 and 40. These filtering arrangements are of conventional type and their purpose is to by-pass fluctuating components appearing in the potential between the conductors 36 and 40, so that these fluctuating components will not flow in the load circuits supplied thereby.

The first voltage amplification stage 20 includes a triode 50 having an anode 5i, :3. control electrode 52, and a cathode 53. The input circuit of amplifier stage 20 may be traced from control electrode 52 through a conductor 54, a condenser 55, input terminal II, the external connections between input terminals II and I: (see Figure 2 for example), input terminal l2, conductor 36, a biasing resistor 56, and a parallel by-pass condenser 51, to cathode 53. The output circuit of the first amplification stage 20 may be traced from positive supply line 40 through resistors 42 and .43, a conductor 60, a load resistor 6|, anode 5|, cathode 53, and resistor 56 and its-parallel by-pass condenser 51 to the negative supply line 36.

Fluctuations in the potential occurring across the load resistor 6i because of variations in the current flow therethrough are transmitted 3 through a blocking condenser 62 to the input circuit of the second amplification stage 2|. The amplification stage 2I includes a triode 63 having an anode 64, a control electrode 65, and a cathode 66. The input circuit of the second stage 2! may be traced from control electrode 65 through a conductor 61, a. resistor 68, conductor 36, and a biasing resistor 69 and its parallel by-pass condenser 18 to cathode 66. The output circuit of stage 2| may be traced from positive supply line 48 through resistor 42, a conductor 12, primary winding 13 of a coupling transformer l4, and a condenser 15 connected in parallel therewith, a conductor I6, anode 64, cathode 66, and biasing resistance 69 and its parallel by-pass condenser I8 to negative supply line 36.

The coupling transformer 14 has a secondary winding 11, which is connected in the input circuit of the final power amplification stage, as described hereinafter. The power amplification stage 22 includes an electric discharge device 88 of the twin triode type. The device 88 includes an upper triode, as it appears in the drawing, consisting of an anode 8I, a control electrode 82, and a cathode 83. The twin triode 88 also includes a lower triode, consisting of an anode 84, a control electrode 85, and the same common cathode 83.

The input circuit of the upper triode may be traced from control electrode 82 through a conductor 86, the upper half of secondary winding 11, center tap 81, a conductor 88, a resistance 98, and a conductor 9i to cathode 83. The output circuit of the upper triode may be traced from positive power supply line 48 through the upper half of a secondary winding 92 on a coupling transformer 93, a conductor 94, anode 8|, cathode 83, and conductors 9| and 95 to negative supply line 36.

The input circuit of the lower triode in the power amplification stage 22 may be traced from control electrode 85 through a conductor 96, the lowerhalf of secondary winding 11, conductor 88, resistor 98, and conductor 9| to cathode 83. The output circuit of the lower triode may be traced from positive supply line 48 through the lower half of secondary winding 92, a conductor 91, anode 84, cathode 83, and conductors 9| and 95 to negative supply line 36.

A condenser 98 is connected across the terminals of transformer primary winding 92 for phase shift correction purposes. The transformer 93 is provided with a secondary winding 99 which is directly connected to the amplifier output terminals I3 and I4.-

From the foregoing description, it will be apparent to those skilled in the art that the final. power amplification stage 22 includes two electrical discharge devices, namely, the upper and lower triodes of the twin triode 88, which are connected in what is commonly termed a push-pull circuit. These two triodes are hereinafter described in this application as being connected in phase opposition. This latter phrase is intended to be broad enough to cover not only push-pull circuits, but other similar connections of two discharge devices, which although not true push-pull circuits, operate in an equivalent manner in so far as a control system of the type described herein is concerned. An example of such a circuit is shown in my co-pending application No. 437,561, filed April 3, 1942, Patent No. 2,423,534, dated July 8, 1947.

While no heater filaments or power supply therefor are shown in connection with the triodes 4 52, 63 and the twin triode 88, it'will be readily understood by those skilled in the art that the elements and connections necessary to heat the cathodes of the respective discharge devices may be readily supplied.

A connection is provided from the power amplification stage 22 to the first voltage amplification stage 28 for supplying a bias to the first stage which varies inmagnitlidewith the input to the power stage 22.- The resistance 98 is common to both input circuits of the final stage 22. It will be seen that whenever the potential of either control electrode 82 or 85 is sufliciently positive with respect to cathode 83 to cause a flow ofycurrent through the input circuit, that current, flowing through the resistor 98, produces a potential difference thereacross of a polarity such that the right-hand terminal of resistor 98 is positive, and its left-hand terminal negative, as indicated by the legend in the drawing. The right-hand or positive terminal of resistor 98 is connected through conductors and 36, and resistance 56 to cathode 53 of the discharge device 58 in the first amplification stage 28. The.

in the potential appearing across the resistor 98,

so that only the unidirectional component of that potential is impressed, between the control electrode and cathode of the discharge device 58.

The following table shows, by way of example, values of capacitance and resistance which may be used in the amplifier circuit of Figure 1;

Reference Numeral Quantity 10 microiarads. 5,000 ohms. 20,000 ohms. 20 microfarads. .05 microiarad. 5,000 ohms.

25 mierofarads. .25 megohm.

.05 microiarad. .5 mega 5.00010,000 ohms impedance. .02 microiarad- 500 ohms impedance approx.

25,000 ohms. 3,000-8,000 ohms (designed to match impedance of tube and load). 500 ohms approx. .02 to .05 microfarad. 75,000 ohms. .1 megohm. .5 microiarad.

Operation of Figure 1 When an alternating signal is applied to the input terminals II and i2, it is amplified in the stages 28, 2i and 22, and an alternating current of corresponding magnitude fiows in the secondary winding 99 of coupling transformer 93, and is thereby available at the amplifier output terminals I3 and I4.

As is well known in the art, the current flowing inthe secondary winding of a transformer is substantially opposite in phase to the potential applied to the primary winding. Due to the inherent inductance of the transformer, however, the secondary current is not exactly opposite in phase to the primary potential but lags the primary potential by an additional amount. It is also well known that this tendency of the secondary current to lag increases as the load on the transformer is increased. When an inductively coupled amplifier is used in a control system, such as that described hereinafter in Figure 2, wherein the phase of the output current with respect to the phase of the input potential is material to the operation of the circuit, this lag in the coupling transformers may present serious diiiiculties. For example, if the lag in each of the two coupling transformers exceed 45 degrees, the total lag will exceed 90 degrees. As

explained hereinafter, this might cause operaf tion of the control system of Figure 2 in a sense exactly opposed to the desired mode of operation.

I have overcome these difficulties by connecting the potentials developed across the resistor 30 to the input circuits of the first amplification stage 20. It may be seen that when the current flowing in the secondary winding ll of transformer 14 tends to increase, the unidirectional potential developed across resistor 90 likewise increases. This potential is. employed to bias the input circuit of the first amplifier stage. The polarity of this potential, as applied to the input circuit of the first amplifier stage, is such that it reduces the gain of the amplifier stage thereby reducing the magnitude of the current flowing in the output circuit. This reduction in current fiow in the first amplification stage is of course reflected in the current fiow in the final power amplification stage. The net eifect of this variable biasing arrangement is to tend to stabilize the magnitude of the current flowing in the output circuit with respect to the magnitude of the signal applied to the amplifier input terminals. It has been found that the overall input voltageoutput current characteristic of this amplifier is such that as the input signal increases from zero to a large value, the output current rises rapidly until it reaches a predetermined value, after which a further increase in the input potential produces a very small, if any, increase in the output current. In other words, the arrangement operates to provide automatic volume control. The phase shift between the input poten tial and the output current is thereby limited to the phase shift which occurs at the maximum current value obtained. By properly selecting the values of the various impedances in the circuits, the phase shift may be limited to any desired maximum value.

The automatic volume control arrangement shown also operates to prevent overloading of the discharge devices, which might otherwise occur when a large signal is impressed on the amplifier circuit. If such an overload were not prevented, it might be accompanied by an actual reduction in the current flow through any of the discharge devices when the control electrode thereof became very positive. Such a condition would of course produce an undesirable distortion in the output characteristic of the amplifier.

In any amplifier circuit, regardless of the type of coupling used, a certain amount of phase shift has been found to exist between the input signal and the output signal. This phase shift has two components, one substantially constant and independent of the load on the amplifier, and the other variable with the load. The undesirable effects of the variable component maybe overcome by the automatic volume control arrangement previously described. The constant component may be controlled by proper design of the circuit constants. In the present circuit, the

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condensers "I5 and 93 are provided to correct for. By proper design of Figure 2 Figure 2 shows a temperature control system which is adapted to use the amplifier disclosed in Figure 1. In Figure 2, a valve mechanism I II is driven by a motor generally indicated at I through a gear train I I2. The motor I II is of the split phase type and includes a rotor III and a pair of field windings H4 and II, which are displaced i'rom each other in space phase by ninety electrical degrees. The winding H5 is continuously supplied with electrical energy from a secondary winding I I of a transformer I I1 having a primary windin I II. The primary winding may be connected to any suitable source of alternating electrical energy. The energizing circuit for motor field winding II5 may be traced from the upper terminal of transformer secondary winding H6 through conductors I20 and I2I, winding III,

and a condenser I22 to the lower terminal of secondary winding H6. The motor winding 4 is supplied with alternating electrical energy from the output terminals I3 and I4 of an amplifier I0,

which maybe the same as the amplifier I0 shown in detail in Figure 1. The energizing circuit for motor field winding II4 may be traced from amplifier output terminal I3 through conductors I23 and 'I2I, winding H4, and conductor I24 to output terminal I4. r

The amplifier III of Figure 2 is supplied with electrical energy from another secondary winding I25 of the transformer III. The power supply terminals I5 and It or amplifier l0 are connected to the terminals. of winding I25 through conductors I26 and I21, respectively.

The transformer II! has a third secondary winding I23, which supplies electrical energy to a bridge circuit shown generally at I30. The bridge circuit I30 has a pair of input terminals I3I and I32, and a pair of output terminals I33 and I34. The input terminals I3I and I32 are connected to the terminals of secondary winding I23 by means of conductors I35 and I36, respectively.

' The output terminals I33 and I34 are connected to the amplifier input terminals I I and I2 by means of conductors I31 and I38, respectively.

The upper left arm of the bridge circuit I30, as it appears in the drawing, may be traced from input terminal I3I through a fixed resistance I40 and a variable resistance I 4| to output terminal The upper right arm of bridge circuit I30 may be traced from input terminal I32 through a conductor I42, and a temperature responsive resistance element I43 to output terminal I 33. The resistance element I43 is preferably constructed of nickel or some other material having an appreciable temperature coefiicient of resistance.

The lower left arm of resistance bridge I30 may be traced from input terminal I3I through a fixed resistance I44, a portion of a slide wire resistance I45 and a slider I46 cooperating therewith to output terminal I34. The lower right arm of bridge circuit I30 may be traced from input terminal I32 through a fixed resistance I 41 and a portion of slide wire resistance I45 and slider I43 to output terminal I34.

The resistance element I43 is exposed to the air in a space whose temperature is to be controlled, and the valve mechanism I I 0 controls the supply of a heating fluid to that space. The valve III ment oi the valve mechanism I I0. The slide wire resistance I45 and the slider I46 together constitute a follow-up controller I50.

4 Operation of Figure 2 When the parts are in the positions shown in the drawing, the valve mechanism H is in its half -open position, as is indicated bythe fact that the slider I46 is at a central position with respect to its range of travel along,the resistance I45. Let. it be assumed that the bridge I30 is balanced so that the slider I46 is stationary in the position shown.

Under these conditions, assume that the temperature adjacent the resistance element I43 increases, thereby indicating aneed for the operation of the valve mechanism I I0 toward its closed position so as to. decrease the supply of heating fluid to the space being controlled.

This increase in temperature adjacent the resistance element I43 causes aninc'rease in the resistance of that element, thereby changing the division of the supplied potential between the two upper'arms oi the bridge circuit I30, and causing the potential of output terminal I33 to approach more closely the potential of input terminal I3I. Sincethe potential of output terminal I34 remains substantially equal to the median potential of the supply, a difference of potential exists between the output terminals I33 and I34. This difference of potential is transmitted to the input terminals II and I2 of amplifier I0, and it produces a current flow in the circuit through the field winding I I4 of motor I I I, which is connected to output terminals I3 and I4 of amplifier I0. Field winding II5 of motor III is energized by current flowing from secondary winding IIS through condenser I22. The condenser I22 establishes the phase of the current flowing through winding II5 with respect to the phase of the terminal voltage of secondary winding H6. The phase of the current supplied to winding I I4 is determined by the direction of unbalance of the bridge circuit I30 subject to modification by any phase displacement occurring in the amplifier I0. As previously stated, the phase displacement through the amplifier I0 may be controlled by proper design of the condensers l5 and 98. The design of condenser I22 is therefore coordinated with the design of condensers l5 and 98 so that a quadrature phase relationship is maintained between the currents flowing in the motor windings I I4 and I I 5. In accordance with the well known characteristics of split phase motors, when currents of that character are supplied to the windings, rotation of the motor results in a direction dependent upon the sense with which the current in one winding is displaced in phase from the current in the other.

In the present instance, the relative phase displacement of the two currents, as determined by the direction of unbalance of the bridge circuit I43. At the same time, the slider I34 is driven to the left along resistance I43, thereby causing the potential of output terminal I34 to approach the new potential of output terminal I33. The signal applied to the input terminals II and I2 of amplifier I0 is thereby decreased, and as it reaches zero the motor III stops and the bridge circuit I30 is rebalanced.

It will be readily understoodthat it the temperature adjacent the resistance I43 decreases, the potential of output terminal I33 approaches that of input terminal I32 more closely than the potential of output terminal I34. and that an alternating signal whose phase is displaced 180 de grees from the signal under the conditions previously discussed is applied to the input terminals II and I2 of amplifier III. A current displaced 180 degrees from the current previously described therefore flows in winding II4. Since the phase of the current flowing in winding I I5 is constant, the currents in the two windings are still displaced degrees, but the displacement is now in the opposite sense. That is, current which formerly led the other current by 90 degrees now lag the other current by a similar angle. Therefore, the motor III rotates in the opposite direction, moving the valve mechanism IIO towards open position and driving the slider I46 to the right along resistance I45 until the bridge circuit I30is rebalanced.

A well-known characteristic of split-phase motors of the type described is that the torque available at the rotor is a maximum when the currents supplied to the two stator windings are displaced in phase by exactly ninety electrical degrees. If the currents depart from this optimum displacement, the'motor torque decreases as a function of the angle of such departure. If the departure continues until the currents are in phase or of exactly opposite phases, the torque is zero. If the departure from the optimum displacement continues in the same sense after the torque has been reduced to zero, the torque begins to increase again, but in the opposite direction.

It will therefore be readily understood that if the current supplied by the amplifier I0 to the winding II4 should shift in phase with respect to the current supplied to winding N5, the torque supplied by the motor III would be adversely affected. 4

If the amplifier output current should shift in phase by more than ninety degrees from its intended relationship, the direction of operation of the motor would be reversed. From the preceding description of the operation of the amplifier I0, it should be apparent that the automatic volume control arrangement provided therein prevents "undesirable shifts in the phase of the current supplied by amplifier I0 to motor winding II5.

As previously described, the overall input potential-output current characteristic of the amplifier I0 is such that for small unbalances of bridge circuit I30, the output current or amplifier I0 increases rapidly, but that after a certain amount of unbalance has been reached, the output current increases no further. As a result of this character-istlc, it may be seen that during the rebalancing operation of the system, the current flowing in the winding II4 rapidly approaches zero as the bridge circuit I30 approaches its balanced condition. Therefore, the motor is slowed down as it approaches the balance point of the system, and there is substantially no tendency for the motor to overrun its proper balance position.

Such an overrun might cause the unbalance of the bridge circuit to be reversed and produce a continuous oscillation or hunting of the motor.

While I have shown and described a preferred embodiment of my invention, other modifications thereof will readily occur to those skilled in the art, and I- therefore intend my invention to be limited only by the appended claims.

I claim as my invention:

1. Control apparatus, comprising in combination, a device to be controlled, means including an electrical winding for controlling the operation of said device in opposite senses dependent upon the phase of alternating electrical energy supplied to said winding, means responsive to a condition indicative oi the need for operation of said device for producing an alternating potential of magnitude variable in accordance with said condition and of a phase determined by the direction of departure of said condition from a predetermined value, means for amplifying said potential including a preliminary stage comprising an electrical discharge device and a later stage comprising two electrical discharg devices connected in phase opposition, each said discharge devices having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, means coupling said winding to the output circuits of said later stage, and means for limitin the current supplied to said winding so as to limit the undesired phase shift in said current, said lastnamed means comprising an impedance, means connecting said impedance in at least one of said circuits of said later stage so as to produce across said impedance a unidirectional potential substantially proportional to the current flowing in said one circuit, means connecting the negative terminal of said impedance to the control electrode of said preliminary stage, and means connecting the positive terminal of said impedance to the cathode of said preliminary stage.

2. Control apparatus, comprising in combination, a device to be controlled, means including an electrical winding for controlling the operation of said device in opposite senses dependent upon the phase of alternating electrical energy supplied to said winding, means responsive to a condition indicative of the need for operation of said device for producing an alternating potential of magnitude variable in accordance with said condition and of a phase determined by the direction of departure of said condition from a means for limiting the current supplied by said' amplifier to said winding so as to limit the phase shift accompanying a changing flow of current through. said inductive coupling means, said last named means comprising an impedance, means connecting said impedance in at least one of said circuits of said later stage so as to produce across said impedance a potential substantially proportional to the current flowing in said one circuit, means connecting the negative terminal of said impedance to the control electrode of said preliminary stage, and means connecting the positive terminal of said impedance to the cathode of said preliminary stage.

3. Control apparatus, comprising in combination, a device to be controlled, reversible motor means for operating said device in opposite senses dependent upon the phase of alternating electrical energy supplied to said motor means, a normally balanced electrical network having a pair of output terminals and effective to produce at said output terminals an alternating electrical potential variable in magnitude in accordance with the unbalance of said network and of a phase determined by the sense of said unbalance, first impedance means connected in said network and variable in accordance with the magnitude of a condition indicative of the need for operation of said device to unbalance said network, second impedance means connected in said network and variable by operation of said motor means to rebalance said network, means for amplifying said potential including a preliminary stage comprising an electrical discharge device and a later stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode, and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, means coupling the output circuit of said final stage and said motor means, and means for limiting the current supplied to said motor means by said final stage so as to limit the phase shift in said current occun'ing independently of the direction of unbalance of said bridge, said last named means comprising an impedance, means connecting said impedance in at least one of said circuits of said latter stage so as to produce across said impedance a potential substantially proportional to the current flowing in said one circuit, means connecting the negative terminal of said impedance to the control electrode of said preliminary stage, and means connecting the positive terminal of said impedance to the cathode 01' said preliminary stage.

4. A motor control system, comprising in combination, a motor having a plurality of electrical windings, means for energizing one of said windings from a source of alternating potential fixed in phase, means responsive to a condition indicative of the need for operation of said motor for producing an alternating electrical potential of magnitude variable in accordance with said condition, means for amplifying said potential including a voltage amplifying stage and a power amplifying stage, each said stage including at least one electrical discharge device having an anode, a cathode and a control electrode, means coupling the other of said windings to a circuit including the anode and cathode of said power amplifying stage, and means for limiting the current supplied by said power amplifying stage to said winding so as to decrease undesired phase shifting, said last named means comprising an impedance, means connecting said impedance in the path of at least a portion of the current flowing in the device of said power amplifying stage so as to produce thereacross a unidirectional potential substantially proportional to said current, and, means connecting the negative terminal of 1] said impedance to the control electrode of said voltage amplification stage and the positive terminalof said impedance to the cathode of said voltage amplification stage,

5. Control apparatus, comprising in combination, a device to be controlled, an electrical winding for controlling the operation of said device and effective to cause operation of said device in a manner dependent upon the phase relationship between the voltage applied to said winding and a voltage fixed in phase, means responsive to a condition indicative of the need for operation of said device for producing an alternating potential of magnitude variable in accordance with said condition, means for amplifying said potential including a preliminary stage comprising an electrical discharge device and a later stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, the corresponding circuits associated with said two discharge devices in said later stage having respective common portions, means coupling said winding to the output circuits of said later stage, and means for limiting the current supplied to said winding by said later stage so as to limit undesired phase shifts in said current, said last named means comprising an impedance, means connecting said impedance in one of said common portions so as to produce across said impedance a unidirectional potential substantially proportional to the current flowing in said later stage, and means connecting the negative terminal of said impedance to the control electrode of said preliminary stage.

6. Control apparatus, comprising in combination, a device to be controlled, an electrical winding for controlling the operation of said device and effective to cause operation of said device in a manner dependent upon the phase relationship between the voltage applied to said winding and a voltage fixed in phase, means responsive to a condition indicative of the need for operation of said device for producing an alternating potential of magnitude variable in accordance with said condition, means for amplifying said potential including a preliminary stage comprising an electrical discharge device and a later stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, the corresponding circuits associated with said two discharge devices in said later stage having respective common portions, means coupling said winding to the output circuits of said later stage, and means for limiting the current supplied to said winding by said later stage so as to limit the amount of undesired phase shift, said means comprising an impedance, means connecting said impedance in the common portion of said later stage input circuits, so as to produce across said impedance a unidirectional potential substantially proportional to the current flowing therein, and means connecting the negative terminal of said impedance to the control electrode of said preliminary stage.

.7. In combination, an electronic amplifier including a preliminary stage comprising an electrical discharge device and a later stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode, and a control electrode, an input circuit for each of said last named two discharge devices including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, the corresponding circuits associated with said two discharge devices in said later stage having respective common portions, and current limiting means tor-said amplifier comprising an impedance connected into the common portion of said input circuits so as to produce across said impedance a unidirectional voltage drop substantially proportional to the current flowing in said common portion of said input circuits whenever said control electrodes are at a potential higher than that of said cathodes, and means for applying to the input circuit of said preliminary stage a biasing voltage dependent upon the voltag drop across said impedance, said biasing voltage acting to decrease the potential of the control electrode of said preliminary stage upon an increase in the voltage drop across said impedance.

8. In combination, an electronic amplifier including a preliminary stage comprising an electrical discharge device and a later stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode, and a control electrode, an input circuit for each of said last named two discharge devices including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, the corresponding circuits associated with said two discharge devices in said later stage having respective common portions, the control electrodes of said last named two discharge devices being maintained at potentials-sufllciently close to cathode potential that the application of a signal voltage to the control electrodes causes them to assume potentials positive with respect to their associated cathodes, current limiting 3 means for said amplifier comprising an impedance connected into the common portion of said input circuits so as to produce across said impedance a unidirectional voltage drop substantially proportional to the current flowing in said common portion of said input circuits whenever said control electrodes are at a potential higher than that of said cathodes, and means for applying to the input circuit of said preliminary stage a. biasing voltage dependent upon the voltage drop across said impedance, said biasing voltage acting to decrease the potential of the control electrode of said preliminary stage upon an increase in the voltage drop across said impedance.

9. In combination, an electronic amplifier including a preliminary stage comprising an electrical discharge device and a' later stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode, and a control electrode, a coupling transformer having a primary winding and a center tapped secondary winding, means for energizing said primary winding from the output of said preliminary stage, an input circuit for each of said last named two discharge devices including its control electrode and cathode and one half of the secondary of said coupling transformer, an output circuit for each discharge device including its anode and cathode, the corresponding circuits associated with said two discharge devices in said later stage having respective common portions, and current limiting means for said amplifier comprising an impedance connected into the common portion of said input circuits between the center tap of said secondary winding and the cathodes so asto produce across said impedance a unidirectional voltage substantially proportional to the current flowing between said center tap and said cathodes whenever said control electrodes are at a potential higher than that of said cathodes, means connecting the ne ative terminal of said impedance to the control electrode of the discharge device of said preliminary stage, and means connecting the positive terminal of saidimpedance to the cathode of the discharge device of said preliminary stage.

10. Motor control apparatus, comprising in combination, a motor to be controlled and having only two windings one of which is connected to a voltage source fixed in phase, means including a balanceable network for producing an output voltage whose magnitude and eflective direction are dependent upon the extent and the direction of unbalance said network, means for amplifying said voltage including a preliminary stage comprising an electrical discharge device and a final stage comprising two electrical discharge devices connected in phase opposition, each said discharge device having an anode, a cathode and a control electrode, an input circuit for each discharge device including its control electrode and cathode, an output circuit for each discharge device including its anode and cathode, the input and output circuits of said final stage each constituting a pair of corresponding circuits, means for applying to the input circuit of said preliminary stage a voltage dependent upon the voltage produced by said balanceable network, means for coupling the output circuit of said preliminary stage to the input circuits of said final stage in such a manner that upon said-network being unbalanced said discharge devices or said final stage are rendered conductive in a manner depending upon the direction of unbalance of said network, a coupling transformer comprising a primary winding having two portions each connected into one of said output circuits of said final stage and a secondary winding connected to said other motor winding so that said winding is energized with current reversible in phase depen ding upon the direction of unbalance of said bridge, and feedback means comprising impedance means so connected to both circuits of one of said pair of circuits of the final stage as to have a D. C. voltage drop thereacross whenever the network is unbalanced, and means for employing the voltage drop developed across said impedance means to reduce the voltage supplied to said preliminary stage.

ALBERT P. UPTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITEP STATES PATENTS Number Name Date 1,586,233 Anschutz-Kaempfe May 25, 1926 1,988,370 Brown Jean. 15, 1935 2,086,465 Brown, Jr July 6, 1937 1 2,178,552 Barger et al. Nov. 7, 1939 2,192,022 Wills --Feb. 27. 1940 2,243,140 Weagant May 27, 1941 

